The expression of Attaching and Effacing Escherichia coli (AEEC) virulence factors is a tightly regulated process, and, in some cases, the identification of these factors has been difficult because they are either repressed in vitro or the conditions of expression are unknown. While it is evident that expression of certain virulence factors is strictly associated with human disease, the additional factors present in AEEC strains that are linked to their pathogenic process remain unclear. Lack of a full understanding of how the genes encoding these additional virulence factors are controlled is important, because, without this knowledge, we are unlikely to understand the overall pathogenic properties of AEEC strains. Thus, our objective is to determine how the Long Polar (LP) fimbriae in AEEC strains contribute to pathogenesis and to use these fimbrial-encoding genes as markers to detect virulent strains. The central hypothesis is that, in addition to the already characterized colonization factors (e.g., intimin-mediated adhesion), AEEC strains possess a highly regulated LP fimbriae, that plays a role in the colonization process, and although the genes encoding these fimbriae are widely distributed in pathogenic E. coli strains, some LP fimbriae types are found exclusively in specific AEEC strains. We will test this hypothesis through three specific aims, which are to: 1) Define whether Ler and H-NS act as a selective silencing/anti-silencing defense system that controls LP fimbriae expression in AEEC strains; 2) Identify the regulatory protein(s) controlling LP fimbriae expression in atypical EPEC and determine in a rabbit model the function of LP fimbriae during colonization; and 3) Characterize the distribution of the LP fimbrial gene clusters among AEEC strains and determine whether certain LP fimbrial subunit types are reliable markers of different pathogenic AEEC strains. To accomplish our aims, we will fully characterize the functions of Ler, H-NS, and atypical enteropathogenic E. coli-encoded regulators under in vitro and in vivo (infant rabbit colonization model) conditions and perform a detailed study of prevalence of the lpf genes in specific subsets of pathogenic AEEC strains. Our research work is innovative because it capitalizes on our findings regarding novel colonization factors in AEEC strains and their potential application in therapeutics and diagnostics. The results from studies of the regulatory networks controlling LP fimbriae expression have significance, because we will be able to identify fundamental differences to explain the tissue tropism of different AEEC strains and to determine whether silencing of LP fimbriae is an example of a defense system that AEEC strains have against horizontally acquired genes. In addition, the use of the rabbit model will give us new insight into the pathogenesis and colonization properties of AEEC strains. An understanding of the mechanisms underlying AEEC colonization to the gastrointestinal tract will not only further our knowledge of the pathogenesis of these organisms but also provide opportunities for reducing infection rates and improving treatment options against these biological agents classified as category B pathogens due to their potential use as a food safety threat.